The present invention is directed to integrated circuits. More particularly, the invention provides a system and method for switching between high voltage and low voltage. Merely by way of example, the invention has been applied to a memory system. But it would be recognized that the invention has a much broader range of applicability.
Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.
Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility. Making devices smaller is very challenging, as a given process, device layout, and/or system design often work down to only a certain feature size.
An example of such a limit is performance of a switch that outputs a high voltage or a low voltage. FIG. 1 is a simplified diagram showing a conventional switch for outputting a high voltage or a low voltage. VPPAA represents a high voltage, and VDDAA represents a low voltage. For example, the high voltage ranges from 6 volts to 25 volts, and the low voltage ranges from 1.8 volts to 5 volts. ENHV#AA represents an enabling signal for the high voltage, and ENLV#AA represents an enabling signal for the low voltage. For example, when ENHV#AA is at logic low, the high voltage VPPAA is enabled. In another example, when ENLV#AA is at logic low, the low voltage VDDAA is enabled. A level shifter LSH receives ENHV#AA and converts ENHV#AA to ENHV1#AA. The high voltage level of EHNV1#AA is equal to VPP. Additionally, transistors MP1 and MP2 are high voltage PMOS transistors.
VPPDDAA is the output voltage generated at a node 110. When ENHV#AA is at the low voltage level, the transistor MP1 is turned on, and the node 110 is connected to a voltage source for the high voltage VPPAA. When ENLV#AA is at logic low, the transistor MP2 is turned on, and the node 110 is connected to a node 130 that is coupled to a voltage source for the low voltage VDDAA. The substrates of the transistors MP1 and MP2 both are biased to VPPAA.
The switch of FIG. 1 has various disadvantages. For example, the node 110 is biased to the high voltage VPPAA. When ENLV#AA changes from the high voltage level to the low voltage level, the node 110 is connected to the voltage source for the low voltage VDDAA. But the node 110 is previously set at the high voltage VPPAA, and this high voltage needs be discharged to VDDAA. If the load capacitance is large, the discharge current can be high, which may cause the internal latch-up and/or damage the voltage source for the low voltage VDDAA. In another example, for the low-voltage mode, the node 120 cannot be biased to the ground voltage level by adjusting the voltage source for the high voltage VPPAA. The voltage level at the node 120 should be at least greater than the level of VDDAA subtracted by the voltage drop of a forward biased PN junction in order to prevent forward biasing the PN junctions for source/drain regions of the transistors MP1 and MP2. VTP is the threshold voltage of the transistors MP1 and MP2.
From the above, it is seen that an improved technique for switching is desired.